The use of fossil fuel as the combustible fuel in engines results in the combustion products of carbon monoxide, carbon dioxide, water vapor, smoke and particulate, unburned hydrocarbons, nitrogen oxides and sulfur oxides. Of these above products carbon dioxide and water vapor are considered normal and unobjectionable. In most applications, governmental imposed regulations are restricting the amount of pollutants being emitted in the exhaust gases.
In the past, NO.sub.x emissions have been reduced by reducing the intake manifold temperature, retarding the injection timing, and modifying the injection rate shape. And, the adverse effects on fuel consumption, particulate emissions engine performance have largely been alleviated through improvements in the basic engine design and fuel selection. For example, at the present time smoke and particulates have normally been controlled by design modifications in the combustion chamber, particulates are normally controlled by traps and filters, and sulfur oxides are normally controlled by the selection of fuels being low in total sulfur. This leaves carbon monoxide, unburned hydrocarbons and nitrogen oxides as the emission constituents of primary concern in the exhaust gas being emitted from the engine.
Many systems have been developed for recycling a portion of the exhaust gas through the engine thereby reducing the emission of these constituents into the atmosphere. The recirculation of a portion of exhaust gas is used to reduce NO.sub.x pollution emitted to atmosphere. In a naturally aspirated engine this process is relatively simple. But, with a turbocharged, the recirculation of a portion of the exhaust gas into the intake air becomes more complex because the intake pressure may be higher than the exhaust pressure during operating conditions. In many of such past system a volume of the exhaust gas from the engine was redirected to the intake air of the engine through the turbocharger and/or an aftercooler and to the engine. Such systems caused the premature plugging of aftercooler cores and malfunctioning of the systems. Additionally, with such recirculation system deterioration of the exhaust flow was caused by deposit buildup.
Prior turbocompounding systems typically use two turbines in series to raise the exhaust manifold pressure above the intake air. However, turbocompounded engines operating at low engine speeds operate inefficiently due to the decrease in the pressure ratio across the turbines in series. Prior techniques have coupled the compounded turbochargers to the engine using mechanical, hydraulic, and flexible couplings. Mechanical couplings need to be extremely strong to withstand the inertia of the turbine, thus adding cost to the coupling. Hydraulic couplings may be used but add complication to the system and additional losses of efficiency during engine operation. Flexible elements may also be used but may have a resonance problem due to the overlapping of frequencies of the flexible coupling the engine.
Various approaches have been used to address the adverse pressure gradient issue. For example, throttling valves have been installed in the air inlet, back pressure valves in the exhaust gas, intake manifold venturi tubes, etc. to provide sufficient pressure drop to get the exhaust gas to flow to the intake air. Although this provides the necessary pressure drop to functionally operate an exhaust gas recirculation system several disadvantages, such as, fuel consumption, emissions, and/or performance occur. In particular, exhaust gas systems which utilize a turbocharger have several performance disadvantages, such as balancing between the turbocharger compressor and turbine portions and turbine operating efficiencies.
The present invention is directed to overcoming one or more of the problem as set forth above.